Many radio communication systems transmit data via a radio (or wireless) channel in a fixed bit rate stream. The transfer rate of such bit stream is typically 64 Kbps, and the bit error rate of the fixed bit rate stream is lower than the bit error rate of a variable bit rate stream.
Accordingly, many variable bit rates streams are converted into fixed bit rate streams in order to reduce their bit error rates. FIGS. 1A to 1D illustrate a conventional method for converting bit streams having different bit rate lengths into fixed bit rate streams having a certain bit rate length S. Such method was proposed by the NTT Mobile Communication Network of Japan in June 1995.
FIG. 1A illustrates N bit rate streams (or blocks) b1 to bn which respectively have different lengths as shown in FIG. 1A. Also all the blocks b1 to bn originate from the same starting point. Such fact is conceptually shown in FIG. 1A by the alignment of the leftmost edges of the blocks b1 to bn. However, since the blocks b1 to bn have different bit rate stream lengths, the ending points of the blocks b1 to bn are different from each other. Such fact is conceptually shown in FIG. 1A by the fact that the rightmost edges of the blocks b1 to bn are not aligned with each other. Specifically, in FIG. 1A, the bit rates of the blocks b2, b3, and bn are smaller than the specified bit rate length S, blocks b1 and bn-1 are larger than the length S, and block bn-2 is equal to the specified length S.
To transmit all of the blocks b1 to bn at a fixed bit rate, a portion 10 of the first block b1 is appended to the end of the block b2, and a portion 20 of the block bn-1 is appended to the end of the block bn. (FIG. 1B). Then, a portion 30 of the block b1 is appended to the end of the block b3. (FIG. 1C). Finally, a portion 40 of the block bn is appended to the end of the portion 30 of the block b1 which was previously appended to the end of the block b3. (FIG. 1D). Thus, none of the blocks b1 to bn extend beyond the certain bit rate length S and can be simultaneously transmitted at the same fixed bit rate S. However, all of the blocks b1 to bn still have the same starting points as shown in FIG. 1D.
Even though the conventional method described above converts the bit rates of the blocks b1 to bn into fixed bit rates and reduces the bit error rate, it takes a considerable amount of time to perform the variable length coding (VLC) process required to modify all of the blocks b1 to bn to have the same bit rate length S. Furthermore, a very long block (e.g. the block b1) needs to be divided a plurality of times and stored in the shorter blocks b2 and b3. In such instances, the processing time of the conventional method is further increased.